An aqueous hydrogen peroxide solution is widely used in various fields, for example, for a bleaching agent for paper and pulp or as a component in chemical polishing fluids. In recent years, the aqueous hydrogen peroxide solution has increasingly been used in the electronic industry, for example, as a cleaning agent for silicon wafers and as a cleaning agent in production processes of semiconductors. Accordingly, there is a demand for an aqueous hydrogen peroxide solution of enhanced quality in purity as obtained by minimizing the content of various impurities in the aqueous hydrogen peroxide solution.
Generally, hydrogen peroxide is now produced exclusively by the anthraquinone process. In the anthraquinone process, first, a derivative of anthraquinone, such as a 2-alkylanthraquinone, is hydrogenated into anthrahydroquinone in the presence of a hydrogenation catalyst in a water-insoluble solvent. Subsequently, after the catalyst is removed, the reaction product is oxidized with air to generate the original 2-alkylanthraquinone, and at the same time hydrogen peroxide is produced. The produced hydrogen peroxide is extracted from the oxidation product with water to thereby obtain an aqueous solution containing hydrogen peroxide. This process is generally known as the anthraquinone autoxidation process. The aqueous hydrogen peroxide solution produced by the anthraquinone autoxidation process contains inorganic ion/compound impurities, such as Al, Fe, Cr, Na and Si, attributed to, for example, the materials constituting the apparatus. Therefore, the aqueous hydrogen peroxide solution is subjected to purification operation for removing such impurities to thereby attain a high purity in accordance with the required quality in particular use.
Especially in the electronic industry, an extremely high purity is required for the aqueous hydrogen peroxide solution. It is required that, in the aqueous hydrogen peroxide solution, the content of organic impurities be not greater than 10 ppm and the content of metal ion impurities be not greater than 1 ppb. For the removal of impurities from the aqueous hydrogen peroxide solution, it is generally known to treat with an ion exchange resin, a chelate resin, an adsorption resin or the like. When the removal of impurities is carried out on an industrial scale with the use of such a resin, there is commonly employed the continuous liquid pass method (tower process) which ensures high operation efficiency and high removing ratio.
The thus spent ion exchange resin is generally regenerated by a regenerant. For example, an anion exchange resin is regenerated by packing the anion exchange resin in a tower and sequentially passing an alkali aqueous solution, an acid aqueous solution and once more an alkali aqueous solution through the anion exchange resin tower.
However, this conventional method has a drawback in that the regenerant may be mixed in the ion exchange resin to thereby disenable satisfactorily removing ionic impurities from a charged aqueous hydrogen peroxide solution. Further, this conventional method has another drawback in that, in the layer of ion exchange resin, there are formed channels (this phenomenon known as “channeling”), through which much of the regenerant is passed to thereby cause the contact of the regenerant with the ion exchange resin to become nonuniform with the result that the ion exchange resin cannot be homogeneously regenerated. Still further, this regeneration of ion exchange resin is carried out in a purifier tower having been used in the purification of crude aqueous hydrogen peroxide solution, so that the regenerant remains in the purifier tower and mixed little by little into the purified aqueous hydrogen peroxide solution. Furthermore, this conventional method is disadvantageous in that, during the regeneration of ion exchange resin, the ion exchange resin tower cannot be employed for purification to thereby lower the production efficiency of purified aqueous hydrogen peroxide solution.
In these circumstances, the inventors have made extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that packing a spent ion exchange resin in a regeneration tower and regenerating the ion exchange resin, specifically, regenerating the ion exchange resin by repeating at least twice a step comprising passing an aqueous solution of regenerant through the regeneration tower downward from an upper nozzle of the regeneration tower and passing ultra-pure water through the regeneration tower upward from a bottom of the regeneration tower enables not only producing a regenerated ion exchange resin wherein impurity residues are minimized but also effecting homogeneous regeneration of the ion exchange resin. Further, it has been found that such a regeneration enables avoiding mixing of the regenerant into the purifier tower and enables efficiently accomplishing the purification of aqueous hydrogen peroxide solution without interruption thereof. The present invention has been completed on the basis of these findings.
It is an object of the present invention to provide a method of regenerating an ion exchange resin, which enables minimizing impurity residues.